The present invention relates to a hydraulic brake system provided with a wheel slip control means for use with automotive vehicles. The system comprises a master cylinder pressurizable by a hydraulic force booster. Valve means are provided between the master cylinder and the wheel brakes in communication with the master cylinder. Pressure fluid can be discharged from the wheel brakes through the valve means. The pressure fluid discharged from the wheel brakes is replenishable by pressure fluid from the pressure chamber of the hydraulic force booster or an auxiliary pressure source. The stroke of the brake pedal, during slip control, is reduced.
Conventional multi-circuit brake systems operating on the principle of the dynamic in-flow into a brake circuit and wherein the dynamic pressure corresponds to the pressure also acting on the hydraulic booster piston, have the disadvantage that in the event of a failure of the circuit both boosting and brake effect of the circuit are lost.
Further, other suggested brake systems (P No. 35 07 484.1) comprise a pedal-operated force booster furnished with a booster piston and a booster chamber wherein an auxiliary pressure proportional to the pedal force adjusts itself through a brake valve. These systems comprise a feed step cylinder provided with a step bore wherein a dual step piston is displaceably disposed, with a pressure chamber being provided ahead of the large step of the dual-step piston and a feed chamber being provided ahead of the small step. The pressure chamber is in communication with the one working chamber of the master cylinder. A check valve is provided in the line of communication connecting the booster chamber to the pressure chamber of the feed step cylinder and the control slide of the brake valve cooperating with a valve body through which the brake valve is in communication with the pressure chamber. This known brake system has the disadvantage that it involves comparatively large space requirements because the feed step cylinder is formed as an additional separate aggregate. Beyond that, it requires valves of comparatively complex design the operating safety of which is not always assured. Finally, a brake system of this type is not suitable for use with automotive vehicles that are provided with a traction slip control means.
For the control of the traction slip, it is known in the art to activate the wheel brake of the driving wheel tending to race and to decelerate the wheel to such an extent that it remains within a slip range favorable for the transfer of the driving moment (DE-PS No. 31 40 959; DE-OS No. 32 15 739). This will improve both the transfer of the driving performance to the road and the driving stability and, in the event of a front wheel, the steerability of the motor vehicle.
However, if the driving moment of the motor, after application of the brakes, cannot be reduced very quickly, the energy to be transformed into heat in the wheel brake will be so high that the brake will have to be released or otherwise a very high wear results. The overall efficiency degree of the drive will be adversely affected by this mode of operation.
It is an object of the present invention to so form a hydraulic brake system of the afore-mentioned type that it can be used both as a brake slip-controlled system and as a traction slip-controlled system. Beyond that, the brake system is to operate on valves of simple construction, preferably 2-way/2-position valves, and in the event of a failure of the auxiliary pressure source, it is to remain operable as a multi-circuit pedal-operated brake system. Finally, it is to be so arranged as to permit an automatic error check through pressure control valves of simple construction by pressure check switches such that, for example, upon occurrence of leakage, a partial shut-down is effected by actuating corresponding valves.